1. Field of the Invention
The present invention relates to technologies regarding ferroelectric thin film devices and particularly to methods for manufacturing lead-free niobate-system ferroelectric thin film devices.
2. Description of Related Art
Ferroelectrics are very attractive substances because of their peculiar characteristics (such as very high relative permittivity, and good pyroelectric, piezoelectric and ferroelectric properties). So, various devices (such as ceramic multilayer capacitors, pyroelectric devices, piezoelectric devices and ferroelectric memories) have been developed and put into use utilizing such peculiar properties. Typical ferroelectrics are perovskite materials such as barium titanate (BaTiO3) and lead zirconate titanate (Pb(Zr1-xTix)O3, PZT). Of these, lead zirconate titanates (PZTs) provide relatively excellent polarization and piezoelectric properties and are therefore most widely used.
Lead-containing PZTs are specified hazardous substances. However, because there are currently no suitable commercially available alternative pyroelectric or piezoelectric materials, PZTs are exempt from the RoHS directive (the directive on the restriction of the use of specified hazardous substances in electrical and electronic equipment enforced by the European Union and Council of Europe). However, with the growing worldwide responsibility towards global environment conservation, a strong demand exists for development of pyroelectric and piezoelectric devices using lead-free ferroelectric materials.
Also, with the recent trend toward smaller and lighter electronic devices, there is an increasing need for ferroelectric thin film devices in which a thin-film technology is utilized.
Herein, pyroelectric and piezoelectric thin film devices will be described below as representatives of such ferroelectric thin film devices. Piezoelectric devices utilize the piezoelectric effect of a ferroelectric material, and are widely used as functional devices such as actuators and stress sensors. Actuators generate a displacement or vibration in response to an applied voltage to a ferroelectric (piezoelectric) material. Stress sensors generate a voltage in response to a strain produced in a piezoelectric material. Pyroelectric devices detect light (including infrared light) utilizing the pyroelectric effect of a ferroelectric material, and are widely used as infrared human body sensors, etc.
Examples of piezoelectric devices utilizing a lead-free piezoelectric material are described below. JP 2007-019302 A discloses a piezoelectric thin film device including, on a substrate, a lower electrode, a piezoelectric thin film and an upper electrode. The piezoelectric thin film is made of an alkali niobate-based perovskite dielectric material of a chemical formula (NaxKyLiz)NbO3 (where 0<x<1, 0<y<1, 0≦z<1, and x+y+z=1). A buffer layer of a perovskite crystal structure material is formed between the piezoelectric thin film and the lower electrode. The perovskite buffer layer is highly preferentially (001), (100), (010) or (111) oriented. According to this JP 2007-019302 A, the piezoelectric thin film device utilizing the lead-free lithium potassium sodium niobate thin film exhibits sufficient piezoelectric properties.
Piezoelectric devices have a basic structure of a piezoelectric material sandwiched between two electrodes and are micro fabricated into a beam or tuning fork shape depending on their application. So, micro fabrication processes are important in order to put piezoelectric devices using lead-free piezoelectric materials to practical use.
JP 2012-033693 A discloses a method of processing a wafer having thereon a piezoelectric thin film of a chemical formula (K1-xNax)NbO3 (where 0.4≦x≦0.7). The method includes the first step of ion etching the piezoelectric thin film on wafer in an Ar gas atmosphere and the second step of reactive ion etching the resulting wafer in a fluorine based reactive gas/Ar mixture atmosphere. According to this JP 2012-033693 A, a fine pattern of the piezoelectric thin film can be formed, thereby providing a highly reliable and low cost piezoelectric thin film device.
JP 2012-244090 A discloses a method for manufacturing a piezoelectric film device, which includes the steps of: forming a lower electrode on a substrate; forming, on the lower electrode, a piezoelectric film of an alkali niobate-based perovskite material represented by a chemical formula (K1-xNax)NbO3; and wet-etching the piezoelectric film using a hydrogen fluoride-based etchant and a Cr film etch mask. According to this JP 2012-244090 A, the Cr mask and the lower electrode are not etched by the hydrogen fluoride-based etchant; therefore, the piezoelectric film alone can be selectively etched. Thus, a fine pattern of the piezoelectric film can be accurately formed in a short time.
As described above, niobate-based ferroelectric materials (such as potassium sodium niobate ((K1-xNax)NbO3) are very promising as lead-free ferroelectric materials. In order to put thin film devices using niobate-based ferroelectric materials as alternatives for PZTs to practical use and mass production, it is very important to develop and establish techniques for micro fabricating niobate-based ferroelectric thin film devices with a high dimensional accuracy and at low cost.
However, niobate-based ferroelectric materials are relatively new materials, and their micro fabrication techniques are still under development. In these respects, the above-disclosed manufacturing techniques have the following disadvantages: The dry etching technique of the above JP 2012-033693 A would achieve a high dimensional accuracy. However, this dry etching technique requires an expensive etching apparatus because it is a vacuum process. In addition, the technique has only a relatively low throughput.
The wet etching technique of the above JP 2012-244090 A would achieve a relatively high throughput and therefore have an advantage over dry etching processes in terms of manufacturing cost. However, niobate-based ferroelectric materials are chemically stable and are therefore difficult to fine-etch using an etchant other than hydrogen fluoride-based etchants. Hydrogen fluoride-based etchants require very careful handling for safety reasons and the usable etch masks are limited. These disadvantages increase the manufacturing cost and therefore might impair or offset the above-mentioned manufacturing cost advantage.